Various types of data transmission signals are in common use which conform to one of many recognized standards. The incoming data or input signal is translated to voltage levels which are capable of being processed by decoding circuitry. A commonly used standard to which data transmission signals conform is specified by the Electronic Industries Association and generally called EIA RS-232-C, hereinafter termed the EIA Standard. Although the following discussion particularly considers an input signal conforming to the EIA Standard, other input signals may be accommodated without departing from the scope and spirit of the invention. The EIA Standard specifies that the signal voltage levels are limited to a range between .+-.3 volts to .+-.25 volts. Further, the EIA Standard prescribes that a zero input voltage, corresponding to a power down or an open condition be interpreted as a control off or the negative voltage level. Prior known circuits generally used to detect such signal levels have incorporated linear design techniques which are both process and temperature sensitive requiring critical circuit configurations. Voltage comparators utilizing linear operational amplifiers have usually incorporated predetermined threshold levels. That is, when the input level exceeds the threshold level, positive feedback reinforces the incoming signal and produces an abrupt change in the output signal. A particular disadvantage with such linear comparators is their general inability to implement both digital and linear devices using economical processing techniques. The decoding circuit, while often being implemented with MOS logic elements, is interfaced to the incoming signal with linear devices thus precluding the total use of economical large scale integration. The voltage comparator hereinafter described is digital and thus may be conveniently implemented with MOS field effect transistors and incorporated with the necessary decoding logic in a single integrated circuit device.